The present invention relates to gas turbine engines and, more particularly, to mounting arrangements for turbine nozzles.
Turbine nozzles within a gas turbine engine provide the function of directing and/or re-directing hot gas flow from a turbine engine combustor into a more efficient direction for impinging on and effecting rotation of turbine rotor stages. A nozzle comprises a plurality of radially extending airfoils arranged circumferentially about an engine axis, the airfoils being supported by radially inner and outer circumferential bands. Either the inner or outer band may include some form of flange for coupling the nozzle to a stationary engine mounting structure. In general, a plurality of turbine nozzles is interleaved with a plurality of turbine rotor stages. At least some of the nozzles are supported only at their radially outer band in essentially a cantilever type arrangement since their radially inner band extends adjacent a rotating engine structure to which the turbine rotor stages are attached. The directing process performed by the nozzles also accelerates gas flow resulting in a static pressure reduction between inlet and outlet planes and high pressure loading of the nozzles. Additionally, the nozzles experience high thermal gradients from the hot combustion gases and the coolant air at the radial mounting surfaces.
In common mounting systems, the turbine nozzle may be attached by bolts or a combination of bolts and some form of clamping arrangement to an engine support structure. In some stages, such as the first stage nozzle, the nozzle is attached to the engine stationary structure via a radially inner mount or flange structure coupled to the inner band. The radially outer band is not mechanically retained but is supported against axial forces by a circumferential engine flange. In other stages, such as stage 2 of an engine, the nozzle may be attached at its radially outer band but be free at its radially inner band.
In either design, the use of bolts and clamps at circumferential locations about a nozzle band act as a restriction to the band, which band is hotter than the structure to which it is attached, causing radial bowing of the outer band of the nozzle and stressing of the airfoils attached to the band. Such stressing of the airfoils may lead to formation of cracks in the airfoil trailing edge.